Liquid crystal display elements have come to be used in not only clocks and calculators, but also in various types of measuring instruments, automobile instrument panels, word processors, Personal Digital Assistants, printers, computers and televisions. Typical examples of liquid crystal display methods include TN (twisted nematic) types, STN (super twisted nematic) types, DS (dynamic scattering) types, GH (guest host) types and FLC (ferroelectric liquid crystal) types. In addition, multiplex driving instead of the conventional static driving has become the most common type of driving method. Moreover, simple matrix types, and more recently, active matrix types, have come into practical use.
Liquid crystal materials are required to have various characteristics to accommodate these display and driving methods. Although a wide temperature range is extremely important in nearly all cases, this includes that in which the nematic phase upper limit temperature (TN-I) is sufficiently high, and the melting point (TC-N) or the smectic-nematic transition temperature (TS-N) is sufficiently low.
In addition, co-solubility with other liquid crystal compounds and versatile liquid crystal compositions is also important. If this co-solubility was defective, it became necessary to mix extremely many kinds of liquid crystal compounds in order to avoid the risk of precipitation and phase separation, making compound preparation extremely bothersome and making increased costs unavoidable.
In addition, a sufficiently low driving voltage is also an important characteristic in many cases, and it is necessary for the threshold voltage (Vth) to be low in order to accomplish this.
In addition, rapid response is also an equally important characteristic, and the viscosity of the liquid crystal is required to be as low as possible in order to accomplish this.
In addition, birefringence (Δn) is also an important characteristic. Although various values are required according to the display method, a low value is frequently required in the case of liquid crystal devices having a large cell thickness for easy manufacturing.
Although an extremely large number of liquid crystal compounds have been synthesized in the past in order to satisfy these requirements, not all of the problems were able to be solved. Thus, there is a need for a liquid crystal compound having superior characteristics with respect to each of the above requirements.
In general, liquid crystal compounds are formed from a central skeleton (core) portion and side groups (side chains and polar groups). There are numerous known examples of the ring structure that composes the core portion, such as a 1,4-phenylene group (which may be substituted with fluorine) and trans-1,4-cyclohexylene group, as well as heterocyclic aromatics such as a pyridine-2,5-diyl group and pyrimidne-2,5-diyl group, and saturated heterocyclic rings such as a dioxane-trans-1,4-diyl group and piperidine-1,4-diyl group. However, this ring structure is practically limited to a 1,4-phenylene group (which may be substituted with fluorine), trans-1,4-cyclohexylene group and a small number of heterocyclic aromatics. However, liquid crystal compounds composed of these ring structures alone are currently unable to adequately accommodate the characteristics required of increasingly sophisticated liquid crystal compounds.
Since compounds containing a trans-2,6-trans-decahydronaphthalene group are saturated rings that do not contain hetero atoms such as oxygen atoms or nitrogen atoms, in addition to being expected to demonstrate superior stability, they are also expected to improve liquid crystal properties. However, there have been few examples of trans-2,6-trans-decahydronaphthalene derivatives reported thus far (W. Sucrow and H. Wolter, Chimia, 36, 460 (1982); Mol. Cryst. Liq. Cryst., 95, 63 (1983)), and hardly anything is known regarding their characteristics.